1. Field of the Invention
The present invention relates to a secondary battery and, snore particularly, to a negative electrode for a secondary battery that can enhance a uniform distribution of an electrolyte, thereby preventing the negative electrode from locally expanding.
2. Description of Related Arts
Lithium secondary batteries are rechargeable and compact in size. yet provide a large capacity. Well known as the lithium secondary battery is a lithium-ion battery.
In the lithium-ion secondary battery, a lithium metal oxide is used LIs a positive active material and a carbon-based material is used as a negative active material. In addition, a mixture solution where a lithium salt was dissolved within a mixture solvent of ethylene carbonate and dimethoxy carbonate is used as an electrolyte. The lithium salt is selected from the group consisting of 6-fluoro phosphorus boron lithium (LiPF6) and 4-fluoro boron lithium (LiBF4).
Therefore, the charging operation is realized while lithium ions released from the positive electrode are absorbed into the carbon-based material of the negative electrode, and the discharging operation is realized while the lithium ions absorbed into the carbon-based material of the negative electrode are absorbed into the lithium metal oxide.
The positive electrode is made by depositing a lithium metal oxide paste on a positive substrate, then drying and roll-pressing the same. The negative electrode is made by same processes as those of the positive electrode using carbon-based material paste. Here, each of the pastes is composed of an active material, a conductive material, and a binder. After the roll-pressing process is completed, the electrodes are cut to a desired length. A separator is disposed between the positive and negative electrodes, then rolled having a plurality of turns, thereby obtaining a rolled electrode assembly. The rolled electrode assembly is inserted into a can, then the electrolyte is injected into the can. Finally, a cap assembly is air-tightly mounted on the can.
FIG. 1A is a partially exploded perspective view illustrating a conventional lithium ion battery, as previously described. Referring to FIG. 1A, a lithium secondary battery 10 includes a can 15 and an electrode assembly 14 installed inside the can 15. Here, the electrode assembly 14 is constructed such that a separator 13 is disposed between a positive electrode 11 and a negative electrode 12. A cap 16 is mounted on the can 15.
In addition, it is well known that, when recharging, for example, a 4.1 V battery to 4.2 V, a capacity of the battery is increased by 10%. However, in this case, the negative electrode locally expands and contracts. This causes the life span of the battery to be shortened as the charging and discharging operations are repeated.
Describing more in detail, it is well known that a rolled electrode assembly is contained in an electrolyte within a can. Therefore, when the battery is over-charged by 10% by, for example, the 4.2 V charging operation, upper and lower portions of the rolled electrode assembly around which a relatively large amount of electrolyte is distributed continue their normal reaction without any problems. However, a middle portion of the rolled electrode assembly. around which a relatively small amount of the electrolyte is provided, does not sufficiently react.
The rolled electrode assembly reacts throughout its entire portion until the battery is charged to a predetermined amount. However, as the battery is charged to a full amount, the upper and lower portions of the rolled electrode assembly become over-charged. As a result, the negative electrode deposited with the carbon-based material, in which the lithium ions released from the positive electrode are absorbed, locally expands to an excessive level at its upper and lower portions. Furthermore, the discharging amount of the negative electrode becomes large during the discharging operation. For example, Ad hen the battery is charged to 4.1 V, the surface temperature of the battery is within a range of 41-42 xc2x0 C. at the end of the discharging operation, whereas when over-charged to 4.2 V, the surface temperature of the battery is within a range of 45-46 xc2x0 C.
The above-described local over-charging problems due to the non-uniform electrolyte distribution causes the porous structure of the separator made of polyethylene to become deformed. That is, when observing a SEM picture of the porous structure of the separator, it is noted that the portion of the separator contacting the negative electrode loses its porous structure. This is referred to as a xe2x80x9cshut-downxe2x80x9d of the separator and is caused by both the local expansion of the negative electrode and the heat generated during the discharging operation of the battery.
When the shut-down of the separator occurs, a portion of the rolled electrode assembly corresponding to a portion of the separator where the shutdown occurs cannot react. That is, this portion is left as a non-reaction area where no charging and discharging occurs. This non-reaction area is increased as the charging and discharging operations are repeated many times, reducing the durability of the battery.
For the forgoing reason, there is a need for a battery that can prevent a negative electrode from expanding and contracting when the battery is overcharged by 10 % by the 4.2 V charging operation and can retain 75 % of its capacity after 300-recharging cycles.
To achieve the above need, the present invention provides an electrode for a secondary battery comprising a substrate deposited with an active material and a metal oxide.
The metal oxide may be coated on a surface of particles of the active material of the negative electrode.
The metal oxide and the active material may be powdered, then mixed together, and deposited on the substrate.
Preferably, the metal oxide is selected from the group consisting of magnesium oxide (MgO), aluminum oxide (Al2O3), barium oxide (BaO), and zinc oxide (ZnO).
Preferably, amount of the metal oxide is 1-5 wt %. A particle size of the metal oxide is less than 5 xcexcm.